Avalanche photodiodes (APDs) are often used in optical systems to sense optical data signals. In this regard, an APD is typically a high-speed, highly sensitive semiconductor device that uses the photoelectric effect to convert light to electricity. In operation, photon or light impact from an optical data signal creates hole-electron pairs within the APD, and the APD operates under a high reverse bias condition to enable avalanche multiplication of the holes and electrons. The avalanche action enables the gain of the diode to be significantly increased, providing a much greater level of sensitivity. Thus, the sensitivity of the APD is directly related to and controlled by the bias voltage that is applied across it, i.e., the higher the bias voltage applied, the higher the gain of the APD.
In order to enhance the sensitivity of an APD, it is generally desirable to provide a bias voltage that is close to the breakdown voltage of the APD, but the breakdown voltage varies with temperature and process. Accordingly, to keep the bias voltage within a desired range of the breakdown voltage, it is possible to measure the current flowing through the APD and to adjust the bias voltage based on such current. Accurately sensing the APD current and controlling the bias voltage based on the sensed current may enhance the performance and, specifically, the sensitivity of the APD.
Various conventional circuit designs for measuring the APD current and controlling the bias voltage suffer from accuracy problems and/or provide insufficient and/or after the fact current limiting for the APD, thereby exposing the APD to possible damage from high currents flowing through the APD. Improved circuits for accurately sensing APD current and providing reliable protection from high currents are generally desired.